Aerosols are stable systems consisting of a gaseous (air) medium and miniscule suspended solid and liquid particles. Aerosols are conventionally classified into dusts, mists, and smokes, although a number of systems can refer to both types at once, e.g., dusts and smokes.
Aerosols are formed in two main ways: 1) by dispersion, when fine particles are formed as a result of crushing a solid or atomization of a liquid; and 2) by condensation, when the aerosol particles originate as a result of molecule aggregation.
The first method is predominant in dust generation. In nature, Dusts are the products of mineral and soil erosion, of volcanic eruptions and dust storms. Dusts commonly consist of particles of irregular, sometimes crystalline, shape and constitute polydisperse systems with particles from fractions of a micron (μm) to 100 microns in size. However, coarse particles (over 10 μm) are liable to sedimentation and rapidly settle. Thus the size of aerosol particles proper is restricted to a maximum of about 10 microns with a minimum size of tenths or even hundredth of a micron.
Mists are mainly formed by condensation. Air containing water vapor is cooled below the saturation temperature and the vapor becomes supersaturated. If the air contains fine dust particles, they serve as the sites of condensation and minute liquid droplets appear on them, which afterward may grow due to either persisting condensation or as a result of coagulation. The absence (or a low number density) of solid particles causes inhibition of vapor condensation and, as the gas is cooled, the supersaturation value rises. Finally, at a certain, critical, supersaturation condensation centers nucleate homogeneously in the vapor itself, and further condensation, known as spontaneous condensation, proceeds on them. In this case, the finest particles are produced abundantly, which facilitate their coagulation and, consequently, growth. The range of droplet size in natural mists lies within 0.01 to 10 μm. As the droplets become larger, they show a tendency to fall, i.e., the mist as an aerosol system decays. Mist droplets are initially spherical and remain so after coagulation. Only very large droplets may be slightly flattened or extended during falling.
A further type of mist is formed as a result of chemical reactions. In nature, minute droplets of sulfuric acid are generated in this way. Sulfur oxides, chiefly SO2, ejected into the atmosphere, are re-oxidized to form SO3 which, reacting with atmospheric water vapor, produces H2SO4, whose vapors are at once in a supersaturated state and condense as droplets. These, in turn, may react with other substances in the atmosphere; for instance, ammonia to produce salts, e.g., ammonium sulfate, settling as submicron solid particles.
Smokes are finely dispersed, extremely stable aerosol systems produced in chemical reactions between gaseous substances that lead to generation of solid particles. In nature, the main source of smokes is combustion. Thus, one way of generating submicron soot particles is the reaction 2CO → CO2 + C proceeding in the flame, where other solid particles, including benzapyrene, are generated. The particle size characteristic of smokes is from 0.001 to 5 μm; the shape may be either crystalline or some other.
Human activities, that is, mining, industry, mistreatment of soil, explosions for military and peaceful purposes have substantially increased the amount of particles ejected in the atmosphere, forming aerosols in the environment. In some industrial regions, dust loading is so high that the solar radiation attenuated, and at a high air humidity (e.g., in north-western Europe) a persistent mist known as smog is formed.
The most widespread mechanism for aerosol generation is vapor condensation with the gas continuum. In a pure vapor-gas mixture in which there are no particles, molecular aggregates (nuclei) of the vapor itself, arising as a result of a high supersaturation, become the sites of condensation. The nucleus, which may give rise to a droplet growth at a given supersaturation, is said to be of a critical size. This critical size is the smaller, the higher the supersaturation. Figure 1 demonstrates the critical radius of nucleating droplets versus the supersaturation value for water (1) and dioctyl phthalate (2). Critical supersaturation depends on the properties of the substance being condensed:
where σ and ρ are the surface tension and the condensate density, respectively; , the molecular weight of the substance; p and p∞, the vapor pressure over the nucleus surface and over a plane liquid surface at the ambient temperature T.
If the vapor contains particles whose surface is wetted by the condensate, the process proceeds at a lower supersaturation. A still lower supersaturation is needed if particles consist of a substance soluble in the condensate.
The presence of charged particles in a vapor-gas medium substantially affects the value of (p/p∞)c (Figure 2). The charge sign has been shown experimentally to play a significant role: negative ions are more efficient.
The above effects are all used for artificial generation of aerosols. The simplest technique is to inject a vapor into a cool air stream, either pure or containing particles; this produces a polydisperse aerosol. Generation of monodisperse aerosols needs a more intricate technology. An example is the Sinclair-La Mez generator (Figure 3), which facilitates the production of aerosols with particles of about 1 μm, differing from one another by no more than 10%, by thoroughly controlling vapor condensation on appropriate nuclei.
A specific feature of aerosols is the spontaneous coagulation of particles, resulting in sticking together, growing in size, and, finally, sedimentation of the contacting particles. The main factor underlying coagulation is the Brownian motion of particles. For coagulation of monodisperse aerosols, the following equation holds:
where n and n0 are, respectively, the number of particles in a unit volume at time t and those present at t = 0, , the gas constant; T, the temperature; N, the Avogadro number; η, the medium viscosity ; and, s, the ratio of the particle effective range to the radius of the particle itself (if the particles coagulate only at s = 2r). If the molecule mean free path is commensurable with the particle radius r, the right-hand side of the equation is multiplied by the Cunningham correction ( /r)), where A is a constant equal to 0.9 for smoke.
For polydisperse aerosols, coagulation occurs at a higher rate. In addition to temperature and pressure, it is also affected by active mixing, including the supersaturation of acoustic fields, and by electric charges on the particles.
Studying aerosol generation, coagulation, and other processes, as well as the practical utilization of aerosols, makes it necessary to measure their basic parameters which, first and foremost, include particle size and shape, particle number density and mass fraction in a gas (see Drop Size Measurement).
The simplest and most frequently used devices for drop size measurement are impactors and thermal and electric precipitators. Impactors employ the inertia effects: the finer the particles, the higher the velocity to be imparted to an aerosol flow for a particle to be captured by the surface. Precipitators employ the effect of thermophoresis and electrophoresis, which have different effects on particles of different sizes and make possible that sizing of the particles.
Currently the term “aerosol” also covers gas-liquid mixtures with large droplets, used in the household, in industry and in agriculture. The mixtures are produced by liquid atomization and injection of this dispersed liquid in the gas flow. These mixtures are unstable since large droplets fall rapidly. The generators of such aerosols are simple in design, easy to handle and possess a high capacity which favor their use in agriculture for pesticide distribution over widespread areas such as fields and meadows.
Green, H. and Lane, W. (1964) Particulate Clouds: Dusts, Smokes, and Mists, 2nd ed., London.